The present invention relates to the electroplating of chromium and its alloys.
Commercially, chromium has been plated from aqueous chromic acid baths prepared from chromic oxide (CrO.sub.3) and sulphuric acid. Such baths in which the chromium is in hexavalent form present a considerable health hazard as a result of the emission of chromic acid fumes. In addition, the baths are highly corrosive and it has proved difficult to plate chromium alloys.
The above-entitled application Ser. No. 637,483 describes and claims a chromium or chromium alloy electroplating solution in which the source of chromium comprises an aqueous solution of a chromium(III) thiocyanate complex. The specification further describes a process of plating chromium or a chromium containing alloy comprising passing a current between an anode and a cathode in said electroplating solution. In a preferred form, the chromium(III) thiocyanate complex consists of an aqueous solution of an aquo chromium(III) thiocyanate complex or mixture of complexes having the general formula: EQU ((H.sub.2 O).sub.6-n Cr.sup.III (NCS).sub.n).sup.3-n, where
n=a positive integer from one to six.
Note that subscripts are always positive or zero, but superscripts may be positive, negative or zero. Complexes of this type are well-known. Chromium(III) species in solution are generally octahedral with six ligands coordinated to the chromium atom. These ligands occupy and define the inner coordination sphere of the chromium atom and are inert inasmuch as they exchange very slowly with free ligands in the solution; e.g., the exchange reaction: EQU (Cr(H.sub.2 O).sub.5 (NCS)).sup.+2 +*(NCS).sup.- .fwdarw.(Cr(H.sub.2 O).sub.5 *(NCS)).sup.+2 +(NCS).sup.-
is very slow (the * represents a "tagged" ion). It is the slowness of reactions of this type which complicates the chemistry of chromium(III) and necessitates equilibration of solutions at high temperatures. See the book by Basolo and Pearson entitled, Mechanism of Inorganic Reactions: Study of Metal Complexes in Solution, published by Wiley.
The linear thiocyanate anion NCS.sup.- has unique catalytic properties; it is able to coordinate surfaces to metal ions through its nitrogen atom and to metal surfaces through its sulphur atoms; and its electron density is extensively localized across the three atoms.
The thiocyanate anion is believed to catalyze the electron transfer reaction: EQU Cr(III)+3e.sup.- Cr(O)
through the formation of multiple-ligand bridges between a thiocyanate Cr(III) complex and the surface of the cathode. The electro-active intermediate can be identified as: EQU Cr(III)-NCS-M
where M is the metal surface of the cathode which is Cr(O) after an initial monolayer of chromium is plated. The "hard" nitrogen coordinates to the Cr(III) ion and the "soft" sulphur to the metal surface of the cathode. Multiple-ligand bridging by thiocyanate in the electro-chemical oxidation of chromium(II) at mercury electrodes is described in Inorganic Chemistry 9, 1024 (1970).
Said application Ser. No. 833,635, entitled "Method and Composition for Electrolating Chromium and its Alloys and the Method of Manufacture of the Composition," describes and claims a chromium or chromium alloy electroplating solution in which the source of chromium comprises an aqueous solution of a chromium(III) thiocyanate complex, the ratio in the solution of total chromium(III), i.e., both free and complexed, to total thiocyanate, i.e., both free and complexed, being 1:6. It should be noted that with a chromium to thiocyanate ratio of 1:6 in the electrolyte, the equilibrated mixture with contain chromium(III) thiocyanate species with less than six thiocyanates coordinated to the chromium. For example, if the hexathiocyanatochromate(III) anion (Cr(NCS).sub.6).sup.3- is equilibrated at high temperatures, the predominate chromium solution species will be (Cr(H.sub.2 O).sub.5 (NCS)).sup.+2 and (Cr(H.sub.2 O).sub.4 (NCS).sub.2).sup.+. The same result can be achieved by heating a 1:6 mixture of (Cr(H.sub.2 O).sub.6).sup.3+ and NaNCS. These above applications further describe a process of plating chromium or a chromium containing alloy comprising passing a current between an anode and a cathode in such a solution.
The chromium(III) thiocyanate complex plating solutions described in the above-mentioned applications do not produce the serious health hazard present during plating by the conventional chromic acid bath and, additionally, they do produce an effluent that is easier and safer to dispose of. These plating solutions have many other advantages, including low material cost, greater electrical efficiency and very low corrosion of capital equipment. The deposited chromium is micro-crack free and is capable of being bent without cracking. Further, it has proved possible to plate alloys of chromium by incorporating metal salts in the solution.
The presence of chromium and thiocyanate in the solution in the ratio 1:6 permits the chromium(III) thiocyanate complex to be prepared by equilibrating a commercially available hexathiocyanatochromate(III) salt. This also has the advantage that the concentration of the ion (Cr(H.sub.2 O).sub.6).sup.3+ in the plating solution is maintained at a low level. The presence of (Cr(H.sub.2 O).sub.6).sup.3+ is thought to produce black nonmetallic deposits at low current densities.
Plating chromium from an organic solution containing thiocyanatopentaamine chromium(III) complexes, i.e., (Cr.sup.III (NH.sub.3).sub.5 (NCS)).sup.+2, has been suggested by Levy and Momyer in an article in Plating, November 1970, pp. 1125-1131. However, in this article, the authors state that no deposition was possible using an aqueous solution.
An article in the Journal of Electrochemical Society, "Electrochemical Science," October 1971, Volume 118, No. 10, pp. 1563-1570, by Levy and Momyer describes the deposition of chromium from hexaminechromium(III) formate, i.e., Cr.sup.III (NH.sub.3).sub.6 (HCO.sub.2).sub.3 in an organic solvent (acetamide/formamide which is not a thiocyanate complex). In this article, Levy and Momyer state with reference to the plating bath comprising an organic solvent containing thiocyanatopentaamine chromium(III) disclosed in their 1970 article referenced above that "the baths were unstable during prolonged electrolysis" (page 1564, column 1). Also in the 1971 article, Levy and Momyer suggest the addition of small amounts of thiocyanate to form aquothiocyanatoamine chromium(III) complexes to overcome the effect of water present as an impurity (400 ppm) in the organic solvent. Clearly, there is no suggestion that chromium could be plated from aqueous solutions in the above-referenced articles by Levy and Momyer.
Chromium has been plated from chromium chloride (CrCl.sub.3.6H.sub.2 O) contained in dipolar aprotic solvent (such as dimethylformamide) and water (see U.K. Patent specification No. 1,144,913). The ability to use chromium chloride has been desired for many years because it is a readily available, cheap, trivalent salt.
However, such solutions possess limitations which hindered their industrial acceptance. In particular, parts of complex shapes could not be plated satisfactorily and the poor electrical conductivity of the solution due to the presence of the dipolar aprotic solvent required a power supply capable of supplying up to twenty volts. Reduction in the quantity of the dipolar solvent resulted in an unstable bath. In addition, the solution was relatively expensive. The plating solution also contained between 0.5 and 1.5 M chromium ions (a relatively high concentration). Also, there are health hazards associated with the use of dipolar solvents, e.g., dimethylformamide. Thus, these solutions have not been commercially successful.
U.S. Pat. No. 3,917,517, claiming priority from United Kingdom Patent Application No. 47424/73, describes a chromium or chromium alloy electroplating solution comprising chromic chloride or sulphate having hypophosphite ions as a supplement to or replacement of the dipolar aprotic solvent.
German Offenlegungsschift Nos. 2612443 and 2612444, claiming priority from United Kingdom Patent Applications Nos. 12774/75 and 12776/76, respectively, describe an aqueous solution comprising chromic sulphate having hypophosphite or glycine ions as "weak complexing agents." In addition, the solutions described in these patents require chloride or fluoride ions, respectively.
United Kingdom Patent specifications Nos. 1,455,580 and 1,455,841 describe another approach that has been used to deposit chromium from aqueous solutions of trivalent salts. In these patents, the source of chromium ions was chromic chloride, sulphate or fluoride. In addition, bromide ions, ammonium ions, and formate or acetate ions are stated to be essential.
However, none of the patent specifications describe the use of thiocyanate with its unique catalytic properties.